Earlier this month Google announced that video ads running on YouTube have a chance to be seen 91% of the time. That's great, and a lot better than the 46% of video ads that Google runs outside of YouTube that never have a chance to be seen. But advertisers would like to be able to check Google's and every other publisher's math.

Deep-pocketed brands such as Kellogg are pulling back budgets from major publishers, including Google-owned YouTube and Facebook, that won't let brands bring in third-party viewability companies to verify how many people may have actually seen their ads.

Ad viewability has become a chief concern for many marketers as they realize that many of the ads they're buying online never even had a chance to be seen. In December 2014 Google said that 56% of the web's banner ads are never seen. Evidence like that has led companies like GroupM and Unilever to demand that publishers only charge them for ads that had a 100% chance of being seen.

While there are disagreements over whether 100% viewability is feasible, many marketers believe that they should at least be able to hire independent companies to check the viewability numbers and enable brands to compare viewability rates on an apples-to-apples basis across multiple publishers.

"Third party verification is a critical component to responsively measure ad viewability. The vast majority of publishers agree to it and the few that don't will be adversely impacted with significantly reduced spend or be completely removed from our preferred vendor list," said GroupM's chief investment officer Rino Scanzoni in an emailed statement.

Some publishers, including YouTube and Facebook, have already been impacted.

Kellogg Co. won't buy YouTube because Google doesn't permit third-party viewability measurement, Jim Kiszka, senior manager-paid digital media, North America, said during the question-and-answer portion of a panel on fraud in digital advertising at the Association of National Advertisers Media Leadership Conference in Hollywood, Fla. in March.

"We've always been clear with all partners across the digital ecosystem that as a principle we want to measure our investment and if we can't measure it, it's hard to justify the investment," said Aaron Fetters, director of Kellogg's global insights and analytics solutions center, in a recent interview.

Mr. Fetters declined to discuss instances where Kellogg has pulled budgets from specific publishers over viewability verification issues, but said that Hulu and Yahoo have been actively working with the marketer to make third-party viewability verification possible for ads running on their services.

Kraft Foods Group has also stopped spending with some publishers that won't allow third-party viewability verification, said the company's VP of media, data and CRM Bob Rupczynski. "We will put dollars behind the folks that we know we can measure, we know what the expectations are, and we will consolidate spend where we know we have validation," he said.

Mr. Rupczynski wouldn't say which publishers Kraft has stopped putting dollars behind over the viewability verification issue but did say that no publisher was so big as to be excused. "We hold all publishers accountable equally. We don't have a sliding scale," he said.

One major marketer that is ranked as one of Ad Age's top 100 brands by U.S. ad spend has reduced its spending on Facebook ads, in part, because the social network won't permit independent viewability verification. That marketer has also cut its spending with one publisher that wouldn't allow verification by double-digit millions of dollars and increased its spending with another publisher that did allow third-party viewability verification by double-digit millions of dollars, according to a person with knowledge of the matter.

It's not just about third-party verification. More importantly to advertisers, those third-party companies would be able to use the same method for measuring ad viewability across different publishers -- who may have their own disparate methods -- and give advertisers a standardized look to compare how viewable their ads are on which sites and allocate their budgets accordingly.

"I'm sure they're not out there lying about the number, but I don't know the methodology behind the numbers they're giving me. And more importantly they're using one methodology that is totally a black box, I'm using a different one on other publishers, so I can't compare publisher-to-publisher…. Not being able to be transparent and accountable apples-to-apples across publishers is a very difficult thing that we're dealing with right now," Mr. Rupczynski said.

But even independent apples-to-apples comparisons face their own complications because even independent viewability firms may use different methodologies, as The Wall Street Journal has documented. Still, marketers aren't willing to sit on the sidelines until things work themselves out.

"The direction things are going is if the advertisers insists on a viewable measurement service, in the long run they'll get it," said Pivotal Research Group analyst Brian Wieser. He added that many advertisers may be satisfied with the publisher-provided viewability stats because they understand there's seemingly no way for them to prove without a doubt that an ad was seen.

"What we desire is a consistent way to measure the investments we place as marketers. We're not yet today at a place where we have a consistent set of independent metrics," Mr. Fetters said.

Google and Facebook aren't shying away from the viewability discussion. As mentioned earlier, Google has published viewability stats for the ads it serves around the web and on YouTube, and the Media Rating Council has accredited the company's Active View product that brands can use to buy and measure viewable impressions.

Facebook is also getting on board. Last year the social network introduced a viewability requirement for its ads, though it fell short of the MRC's "50% in view for at least one second" desktop standard. And now it's working with the MRC to get accredited and develop a viewability standard for mobile ads, which account for the majority of Facebook's revenue.

But Google and Facebook remain the biggest hold-outs in terms of publishers blocking advertisers' abilities to verify their online ad viewability rates. Yahoo, AOL, eBay, Hulu, Fox, NBC, ABC, CBS, Turner, The New York Times and The Wall Street Journal are all said to support third-party viewability verification.

There appear to be a couple reasons for the standstill. For some publishers there are concerns that letting third-party firms monitor ad viewability would slow down page load speeds because those outside companies would need to place pieces of code on the publishers' sites that result in more data that needs to be downloaded for a page to render, which can be particularly problematic on a smartphone using a weaker-than-wifi cell signal. And Google's and Facebook's current hesitation seems to fit with both companies' reputations for being extremely protective of their data and not wanting outside companies accessing it without strict measures in place.

"Viewability has long been a concern for our clients, which is why we've supported industry-wide efforts and developed MRC-accredited technology, Active View, to measure and buy based on viewability across our products. Our partners have been very receptive to our focus on viewability and we're continuing to work with them to make sure their measurement needs are addressed," said a Google spokeswoman in an emailed statement.

"We are committed to providing value for advertisers -- we don't want them to spend a dollar with us if it isn't driving value. That is why we use viewed, not served, impressions to measure ad delivery across desktop and mobile. We are also working with the MRC to get accreditation for this approach because we want marketers to have full confidence in their ads being seen. Especially on feed-based, increasingly mobile-centric platforms, third-party verification view tags often slow down consumer experience and don't always report views consistently," said a Facebook spokeswoman in an emailed statement.

Not all marketers are taking such a hardline stance on viewability verification. For example, Wendy's continues to buy ads from Google even if the company can't fact-check how many people may have had a chance to see them.

"More measurement is good, and third party measurement is best, but beyond that we find the inventory at Google to be valuable, so no concerns here that are making us doubt working with them," said Wendy's VP of digital and social media Brandon Rhoten in an email.

Contributing: Jack Neff

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Tim Peterson

Tim Peterson covers digital media for Advertising Age out of the publication's Los Angeles bureau. He previously reported on social media and ad tech for Adweek and worked as a reporter handling the digital marketing beat at Direct Marketing News.